Sporadic CJD - believed to be caused by spontaneous, non-inherited mutations in the prion gene, or spontaneous conversion of the prion protein to the abnormal, disease-causing configuration;

Variant CJD - believed to be caused by ingesting prion-contaminated animal products; and

Familial CJD - caused by inherited mutations in the prion gene.

CJD is classified as one of the above disease types based not only on the (presumed) route of infection, but also based on the person's age, her or his disease symptoms, and especially the appearance of the brain in post-mortem examinations. [2] The likely presence of "strains," or different forms of prions, and oustanding questions regarding what species can contract TSE diseases from what sources, however, have led some to question whether CJD disease classification is as straightforward as the above list might suggest. [3]

Sporadic CJD from TSE-infected Meat?

In February 2004, Italian researchers published a study that identified "a novel form of bovine spongiform encephalopathy (mad cow disease) in two seemingly healthy cows." The "molecular signature of the newly discovered strain, combined with its corresponding brain deposition and disease pathology bear striking similarities to sporadic Creutzfeldt-Jakob disease (sCJD) in humans." [4] The results suggest that there may be more than one "strain" of prion that causes BSE in cows. Moreover, it raises the possibility (but does not prove) that some cases of sporadic CJD in humans might be due to prion-infected meat. [5]

Earlier research at the University College of London, designed to prove the mad cow-variant CJD connection, used transgenic mice engineered to have so-called "humanized" brains (brains with human proteins). After being injected with BSE-infected cow brains, one strain of the "humanized" mice developed a disease similar to human variant CJD, as expected. But in a different "humanized" mouse strain, most of the BSE-injected mice developed a disease just like human sporadic CJD. One conclusion from the study, according to lead researcher Professor John Collinge, is not "that all or even most cases of sporadic CJD are ... a result of BSE exposure, but some more recent cases may be. The incidence of sporadic CJD has shown an upward trend in the UK over the last decade. ... Serious consideration should be given to a proportion of this rise being BSE-related. Switzerland, which has had a substantial BSE epidemic, has noted a sharp recent increase in sporadic CJD." [6]

CJD and Alzheimer's Disease

Other studies have challenged the assumption that sporadic CJD is a rare, "one in a million" occurrence. In 1989, a Yale University team performed autopsies on the brains of patients diagnosed with Alzheimer's disease and found that 13 percent had CJD. A similar investigation found three of 12 patients diagnosed with Alzheimer's to actually have a TSE disease. A larger University of Pittsburg study "found a misdiagnosis rate of 5 percent, and estimated there may be 200,000 cases of CJD in the U.S. each year which are misdiagnosed as Alzheimer's." [7][8][9]